For future telecommunications, the so-called asynchronous transfer mode (ATM) is currently being investigated and developed. This is a fast packet switching technique; the individual packets are referred to as "cells". In principle, the present invention is also applicable to conventional (slow) packet switching techniques.
In conventional time-division multiplexing, now also referred to as "synchronous transfer mode", each message to be transmitted is divided into fractions of equal length (mostly 8 bits) and sent on, together with fractions of other messages, in a data stream in predetermined equidistant time slots. Consequently, all fractions belonging to the same message are on the way for the same time, arrive at the receiver in the same order and with the same spacing, and are identifiable as belonging together by their temporal position in the data stream. This technique is suitable mainly for transmitting analog signals in pulse-code-modulated form.
Packet switching has been used for data transmission for a long time. In packet switching, the data are fitted into empty places of a data stream as they accrue. As the data are not identifiable by their temporal position in the data stream, they are combined with a header to form a packet. The packet header contains all information required to bring the message part to the correct receiver and to correctly assign it to the other message parts arriving there. In principle, each packet could then follow its own path through the telecommunication network. Very early, however, the virtual circuit approach was adapted, in which all packets belonging to the same message follow a path through the entire network which is defined by the first packet. With this solution, the packet headers, which represent an additional load on the network, can be kept shorter, because each of them must only contain the information for one link and can be provided with the information for the next link with the aid of connection tables while being on its way. In addition, this solution prevents packets belonging together from overtaking each other in a simple manner. Thus, the data are certain to arrive at the receiver in the same order in which they were sent out at the transmitter.
There are various reasons to depart from this rigid linkage of the cells of a message to a selected path, at least in the switching network of an exchange. An essential feature of the ATM technique is that random variations in the traffic volume of individual connections are permitted. Where many independent connections are combined, a certain compensation for these variations occurs. A further compensation and, thus, quite a uniform network load results if the individual cells of a connection do not follow the same path but are distributed to all possible paths, particularly within the switching network of an exchange. This eliminates the need for traffic measurements within the switching network and for the devices necessary for such measurements.
Such connections, as opposed to virtual connections, are referred to as "connectionless". It is necessary, however, to restore the original sequence of the cells on reception of the message in the receiver at the latest, because overtaking cannot be ruled out. If a connection is routed as a "connectionless" connection only within an exchange and as a virtual connection outside the exchange, the restoration of the sequence, commonly referred to as "resequencing", will advantageously be effected at the output of this exchange.
PCT Patent Application EP 89/00941, which was not published prior to the priority date of the present application, (and which corresponds to commonly assigned U.S. application Ser. No. 07/566,038 filed Aug. 9, 1990) proposes as a solution to additionally delay each cell by a fixed time with respect to its entry into the exchange in a manner known per se for virtual connections, and to subsequently output this cell. Thus the correct sequence is automatically restored.
As a rule, however, the requirement to keep the delay constant, i.e., to compensate for "delay jitter", is not imposed. To meet this requirement, "time stamps" have to be added to the cells, which, in turn, requires a central clock and a network for exactly distributing the time.